Optical fibre transmission systems are currently going through a tremendous development, and numerous new applications appear within the area of multi-channel optical communications. These new applications are typically based on new functionalities in the optical fibre components, and one of the interesting developments within the past 4-5 years has been the appearance of “photonic crystal” or “photonic bandgap” fibres as described in T. A. Birks et al., Electronics Letters, Vol.31 (22), p.1941 (October 1995), and J. C. Knight et al. Proceedings of OFC, PD3-1 (February 1996). Photonic bandgap fibre typically involves a dielectric structure with a refractive index that varies periodically in a cross-section perpendicular to the fibre length axis. The period is in the order of an optical wavelength, and the guiding mechanism due to the photonic bandgap effect is fundamentally different from the total internal reflection, which is the basic principle according to which the optical standard fibres work. According to the Bragg diffraction principle that is used in photonic bandgap fibres, radiation within certain wavelength intervals can only propagate in the longitudinal direction with essentially no lateral leakage.
Photonic bandgap fibres—and photonic crystal fibres—are a special class of the fibres known in literature as micro-structured fibres, which may all exhibit waveguiding properties that are unattainable using conventional fibres. For this reason, intensive research has recently been directed towards this new field as described in a number of references, such as WO 99/64903, WO 99/64904, and Broeng et al. Pure and Applied optics, 1999, pp.477-482, describing such fibres having claddings defining Photonic Band Gap (PBG) structures, and U.S. Pat. No. 5,802,236, Knight et al. J. Opt. Soc. Am. A, Vol.15, No.3, 1998, pp.748-752, and Monro et al. Optics Letters, Vol.25, No.4, 2000, pp.206-208, defining fibres, where the light is transmitted using modified Total Internal Reflection (TIR).
The mentioned descriptions of prior art within the field of micro-structured optical fibres present several aspects of advantageous design of core and cladding regions and the resulting optical properties of the fibres. Many of the descriptions have included fully two-dimensionally periodic cladding regions—for obtaining the photonic bandgap effect—or more relaxed requirements (including the possibility of random hole distribution) needed for the formation of total-internal-reflection like waveguiding in high-index core micro-structured fibres. However, the present inventors have realised that a new class of micro-structured photonic crystal fibres (which we will in this application refer to as semi-periodic fibres) may enhance the broad possibilities of improved functionality even further, as we will describe in the following text.
It is well-known to those skilled in the art that a dielectric mirror can be made as a periodic structure of stacked dielectric layers with alternating high and low refractive index. Total light reflection for any angle of incidence and any polarisation of the incident light can be obtained by proper design (see J. Lekner, J. Opt.A., Vol.2, 2000, pp.349-352, and references herein for theoretical analysis). Experimental demonstrations can be found in D. N. Chigrin et al., J. Lightwave Technol., vol.17, November 1999, pp.2018-2024. It is now quite natural to imagine that a dielectric waveguide can be constructed by folding the dielectric mirror so that an air (or dielectric) core is surrounded by a totally reflecting layered cladding, which confines the guided light. Described in a cylindrical co-ordinate system whose z-axis coincides with the waveguide axis, the refractive index of the cladding has no angular dependence but has a periodic dependence on the radius. These waveguides are in this patent application denoted as “radial-periodic” structures.
The radial-periodic waveguides have several potential advantages compared to traditional optical fibres, whose light guidance is based on total internal reflection at the boundary between the high refractive index core and the low refractive index cladding. The fact that the light propagates in air rather than in a dielectric such as silica holds out the prospect of reducing both material absorption losses and non-linearities. These phenomena cause serious problems in optical communication systems. Furthermore, it is expected that the multiplicity of design parameters (e.g., refractive indices and dimensions of layers) will make it possible to design waveguides, which closely match even complex design goals as for instance elaborately specified dispersion properties. Finally, the radial confinement of the light can be made much stronger in radial-periodic waveguides than in traditional optical fibres. This reduces bending losses and allows for tighter bends.
Several studies reported in the literature (see J. Marcou et al., ECOC'99 Conference, vol.1, 1999, pp.24-25, M. Ibanescu et al. Science, Vol.289, July 2000, pp.415-419, and T. Kawanishi et al., Optics Express, vol.7, 2000, pp.10-22) illustrate the potential of radial-periodic waveguides. They also indicate that severe practical problems must be expected in connection with the production of hollow radial-periodic waveguides. J. Marcou et al., ECOC'99 Conference, vol.1, 1999, pp.24-25 describes a radial-periodic waveguide design that can be obtained using silica with practically obtainable levels of germanium doping. The most notable characteristics of this design is that the zero dispersion wavelength is shifted downwards to the 850 nm range and that the refractive index of the high index layer of the cladding is only 0.6% higher than the index of the low index layer. It is a disadvantage of radial periodic fibres that significantly larger index contrasts cannot be obtained in a traditional fibre production process relying on doping of silica. It is a disadvantage that this waveguide is not hollow but has an un-doped core with slightly lower refractive index than the cladding layers. The potential advantages of the hollow core waveguides concerning properties such as loss and non-linearity are significantly reduced in this case.
The desirable light guidance in the air core of a hollow waveguide requires much higher index contrasts in the cladding. The theoretical study in T. Kawanishi et al., Optics Express, vol.7, 2000, pp.10-22 is a radial-periodic structure with refractive indices of 1.0 and 2.0, whereas the design described by M. Ibanescu et al. (Science, Vol.289, July 2000, pp.415-419) employs layers with refractive indices as high as 1.6 and 4.6. Experimental verification of hollow waveguides is reported by Y. Fink et al., J. Lightwave Technol., vol.17, November 1999, pp.2039-2041, who transmitted light in the 10 micron region through a 10 cm waveguide made of tellurium and polystyrene. However, in large scale production of radial-periodic hollow waveguides for employment in telecommunication systems, index contrasts of this order of magnitude could turn out to be an insuperable difficulty of large-scale production of semi-periodic hollow waveguides, since it is expected to be very difficult to find the required dielectrics. They must have high index contrasts, low loss, chemical stability and mechanical robustness, and they must furthermore be thermally, chemically and mechanically compatible so that they can be combined in a production process. Moreover it is a disadvantage from an economical point of view that completely new production processes must be developed, if optical waveguides are no longer based on silica.
In connection with the prior art on radial varying fibre cladding structures, it is relevant to note that in European patent application EP 0 810 453 A1 and in U.S. Pat. No. 5,802,236 is described that micro-structured fibres are not limited to fibres with an array of cladding features. EP 0 810 453 A1 and U.S. Pat. No. 5,802,236 describes an alternative micro-structured fibre having circular symmetry, with the core feature surrounded by a multi-layer (exemplarily more than 10 or even 20 layers) cladding, with alternating relatively high and low refractive indices. It is, furthermore, mentioned that the refractive indices and layer thickness are selected such that the structure has a desired effective refractive index profile. For instance, the layer thickness can be chosen such that an inner cladding region has a relatively low effective refractive index, and an outer cladding region that surrounds the inner cladding region has an effective refractive index of value between that of the core region and the inner cladding region. In EP 0 810 453 A1 and in U.S. Pat. No. 5,802,236, it is further described that such a micro-structured fibre can be made, for instance, by drawing from a preform, with the described multilayer cladding formed by e.g., a conventional deposition technique such as MCVD, or by collapsing a multiplicity of glass tubes around the core feature.
It is a disadvantage of the fibre described in EP 0 810 453 A1 and in U.S. Pat. No. 5,802,236, that only homogeneous material is used in the concentric circles forming the alternating relative high and low refractive index cladding, and as described above this results in limited intervals within which the refractive index values of the individual layers may be chosen. It is further a disadvantage that the possibility of using a radial periodic cladding is not considered, such that the radial varying fibres disclosed in EP 0 810 453 A1 and in U.S. Pat. No. 5,802,236 will not allow photonic bandgap guidance of light.
It should, furthermore, be pointed out that EP 0 810 453 A1 also describes a non-periodic micro-structured fibre preform having a silica-rod core feature of diameter 0.718 mm. This rod is surrounded by six silica tubes of inner diameter 0.615 mm and outer diameter 0.718 mm, which in turn are surrounded by more than four layers of silica tubes of inner diameter 0.508 mm and outer diameter 0.718 mm. This preform is overclad with silica tubes selected to yield, after drawing, a desired fibre diameter. It is, however a disadvantage that this fibre design (employing two different hole dimensions) does not have alternating concentrically distributed effective refractive index values, such as it is the case with the semi-periodic fibres disclosed in this application.
The prior art also includes descriptions of waveguides having axially varying structure such as outlined in Patent application WO 00/16141. More specifically WO 00/16141 describes a micro-structured fibre having axial change in density of the cladding layer. controlled through the fraction of the cladding volume that is air or a glass of a composition different from that of the base cladding glass. The axial variation in cladding indices changes the signal mode power distribution, thereby changing key waveguide parameters such as magnitude and sign of dispersion, cut-off wavelength, and zero dispersion wavelength. The description in WO 00/16141 also incorporates cladding layer structures, which contain an array of features, periodic or randomly distributed, comprising a material in place of the pores, and further the core region may be segmented Into two or more portions, for obtaining equivalent index profiles to those normally described in standard fibre technology. In WO 00/16141 it is described how the cladding layer density can be made to alternate from high to low and low to high in adjacent segments along the preform axis (length axis) of the preform by changing the porosity of the cladding layer, and in particular, respective adjacent segments along the preform axis could alternate between a condition in which the cladding layer is essentially free of pores and a condition in which the cladding layer contain pores. The parameters provided in WO 00/16141 describe fibres with pitches from about 0.4 μm to 20 μm, and a typical outside diameter about 125 μm. In the cases, where the filaments are obtained through a glass of different composition compared to the base glass, it is specifically mentioned that if one wishes the filament containing cladding layer to interact with light in the manner of a photonic crystal having a full band gap, the filament size and spacing should be such to accommodate a pitch in the range of about 0.4 μm to 5 μm, and the respective dielectric constants of the matrix glass and the glass comprising the columns of glass contained therein should differ by about a factor of three. Although several segmented core designs are discussed in WO 00/16141, it is a disadvantage that these fibre designs does not have alternating concentrically distributed effective refractive index values throughout the cladding, such as it is the case with the semi-periodic fibres disclosed in this application, and although photonic bandgap guidance is mentioned, no detailed specifications besides the pitches in the range of about 0.4 μm to 5 μm are provided.
Glossary and Definitions:
For micro-structures, a directly measurable quantity is the so-called filling fraction that is the volume of disposed features in a micro-structure relative to the total volume of a micro-structure. For fibres that are invariant in the axial fibre direction, the filling fraction may be determined from direct inspection of the fibre cross-section.
In this application we distinguish between “refractive index”, “geometrical index” and “effective index”. The refractive index is the conventional refractive index of a homogeneous material. The geometrical index of a structure is the geometrically weighted refractive index of the structure. As an example, a structure consisting of 40% air (refractive index=1.0) and 60% silica (refractive index≈1.45) has a geometrical index of 0.4×1.0+0.6×1.45=1.27. The procedure of determining the effective refractive index, which for short may be referred to as the effective index, of a given micro-structure at a given wavelength is well-known to those skilled in the art (see e.g., Joannopoulos et al., “Photonic Crystals”, Princeton University Press, 1995 or Broeng et al., Optical Fiber Technology, Vol. 5, pp.305-330, 1999).
Usually, a numerical method capable of solving Maxwell's equation on full vectorial form is required for accurate determination of the effective indices of micro-structures. The present invention makes use of employing such a method that has been well-documented in the literature (see previous Joannopoulos-reference). In the long-wavelength regime, the effective index is roughly identical to the weighted average of the refractive indices of the constituents of the material, that is, the effective index is close to the geometrical index in this wavelength regime.
By radial periodic fibres is meant fibres whose refractive index may be described as concentric circles. Such fibres may be described as fibres with a one-dimensional periodicity using semi-polar coordinates as understood by those skilled in the art. By micro-structured fibres is meant fibres with localised features (such as holes or holes containing or filled up with a material having a refractive index that differs from the refractive index of the region containing the feature) in the cladding region, which features may have cross-sectional dimensions in the order of 10 μm or less in a cross section perpendicular to an axial direction of the fibre. Thus radial periodic fibres, two-dimensionally periodic fibres (whose cladding features define a two-dimensional lattice structure), random cladding structures (see WO 00/16141) and semi-periodic fibres (to be defined in the following) may all be termed micro-structured fibres.
In the following description the term “non-periodic” is used in connection with arrangements of features in the cladding region. Here, by using the term “non-periodic” is meant that the whole cladding structure of a fibre cannot be periodic in two independent directions- or stated in terms understood by those skilled in the art: the whole cladding structure cannot be generated from a unit cell (unit cells are used to describe periodic structures). In simple one-dimensional structures the unit cell has the size of one period. Likewise, unit cells for two-dimensional periodic structures have the size of the period. When the structure is periodic in two dimensions, the unit cell becomes an area, such that the boundary of the unit cell on one side describes the periodicity in one independent direction, while the boundary of another side of the unit cell describes the period in another independent direction. It should be understood that the term “non-periodic” also covers cladding structures in which the cladding features differs in some property as for example the cross-sectional diameter.
In particular, embodiments of the present invention allow for parts of the cladding structure to be periodic, while the cladding structure as a whole may be non-periodic. Naturally, this may refer to the part of the cladding structure, which is important for the guided modes only. Thus, when the cladding features in relation to the micro-structured cladding region as a whole are arranged in a non-periodic arrangement, the term non-periodic may refer to that the cladding features are arranged irregularly or differ in some property, e.g. diameter, when looking at the whole micro-structured cladding region.